1. Field of the Invention
The present invention relates to a toroidal-type continuously variable transmission which can be used as a transmission unit of a transmission for use in a car, or a transmission for use in various kinds of industrial machines.
2. Description of the Related Art
Conventionally, it has been studied that such a toroidal-type continuously variable transmission as schematically shown in FIGS. 4 and 5 is used as a transmission for a car. In this toroidal-type continuously variable transmission, as disclosed, for example, in JP-A-62-71465U, an input side disk 2 is supported concentric with an input shaft 1, while an output side disk 4 is fixed to the end portion of an output shaft 3 which is disposed concentric with the input shaft 1. Also, at the lateral positions of the extensions of the input and output shafts 1 and 3 between the two disks 2 and 4, there are disposed two trunnions 6, 6 each of which is capable of swinging about a pair of pivot shafts 5, 5 respectively situated at positions along an imaginary plane that is perpendicular to an imaginary line connecting the respective axes of the input and output shafts 1 and 3, and distanced from the intersection of the imaginary plane and imaginary line. This physical relation is hereinafter referred to as xe2x80x9ctorsional relationxe2x80x9d.
In other words, each of the trunnions 6, 6 is formed with a pair of pivot shafts 5 on outer surfaces of the two end portions thereof in such a manner that the pair of pivot shafts 5 are concentric with each other. Therefore, the pair of pivot shafts 5 are disposed in such a manner that they do not cross the center axes of the two disks 2, 4 but extend at right angles to the center axes of two disks 2, 4. Also, the base end portions of displacement shafts 7, 7 are respectively supported on the center portions of their associated trunnions 6, 6 such that the inclination angles of the respective displacement shafts 7, 7 can be freely adjusted by swinging the trunnions 6, 6 about their associated paired pivot shafts 5, 5. On the peripheries of the respective displacement shafts 7, 7 thus supported on their associated trunnions 6, 6, there are rotatably supported power rollers 8, 8, respectively. These power rollers 8, 8 are respectively held by and between the input side and output side disks 2, 4. Each of the mutually opposing inner surfaces 2a, 4a of the input side and output side disks 2, 4 has a cross section consisting of a concave surface which can be obtained when an arc having a center present on the extension of the center axis of the pivot shaft 6 is rotated about the input and output shafts 1 and 3. And, the peripheral surfaces 8a, 8a of the respective power rollers 8, 8, each of which is formed in a spherical-shaped convex surface, are respectively contacted with the inner surfaces 2a, 4a of the input side and output side disks 2, 4.
Between the input shaft 1 and input side disk 2, there is interposed a pressing device 9 such as a loading cam and, due to the pressing device 9, the input side disk 2 can be elastically pressed toward the output side disk 4. The pressing device 9 is composed of a cam plate 10 rotatable together with the input shaft 1 and a plurality of (for example, four) rollers 12, 12 which are respectively held by a retainer 11. On one side surface (in FIGS. 4 and 5, the left side surface) of the cam plate 10, there is formed a cam surface 13 which is a curved surface extending over the circumferential direction of the cam plate 10; and, on the outer surface (in FIGS. 4 and 5, the right side surface) of the input side disk 2 as well, there is formed a cam surface 14 which is similar to the cam surface 13. And, the plurality of rollers 12, 12 are supported in such a manner that they can be freely rotated about an axis extending in the radial direction with respect to the center of the input shaft 1.
When the above-structured toroidal-type continuously variable transmission is in use, in case where the cam plate 10 is rotated with the rotation of the input shaft 1, the cam surface 13 presses the plurality of rollers 12, 12 against the cam surface 14 formed on the outer surface of the input side disk 2. As a result of this, the input side disk 2 is pressed against the plurality of power rollers 8, 8 and, at the same time, due to the mutual pressing contact between the pair of cam surfaces 13, 14 and the plurality of rollers 12, 12, the input side disk 2 is caused to rotate. And, the rotation of the input side disk 2 is transmitted through the plurality of power rollers 8, 8 to the output side disk 4, so that the output shaft 3 fixed to the output side disk 4 can be rotated.
Now, description will be given below of a case where a rotation speed ratio (transmission ratio) between the input shaft 1 and output shaft 3 is changed. At first, when reducing the rotation speed ratio between the input shaft 1 and output shaft 3, the trunnions 6, 6 may be respectively swung about the pivot shafts 5, 5 in a predetermined direction to thereby incline the displacement shafts 7, 7 in such a manner that, as shown in FIG. 4, the peripheral surfaces 8a, 8a of the respective power rollers 8, 8 can be respectively contacted with the near-center portion of the inner surface 2a of the input side disk 2 and the near-outer-periphery portion of the inner surface 4a of the output side disk 4. On the other hand, when increasing the rotation speed ratio between the input shaft 1 and output shaft 3, the trunnions 6, 6 may be respectively swung in the opposite direction about the pivot shafts 5, 5 to thereby incline the displacement shafts 7, 7 in such a manner that, as shown in FIG. 5, the peripheral surfaces 8a, 8a of the respective power rollers 8, 8 can be respectively contacted with the near-outer-periphery portion of the inner surface 2a of the input side disk 2 and the near-center portion of the inner surface 4a of the output side disk 4. In case where the inclination angles of the displacement shafts 7, 7 are set in the middle of the inclination angles respectively shown in FIGS. 4 and 5, there can be obtained a middle transmission ratio between the input shaft 1 and output shaft 3.
Conventionally, the cross-sectional profile (hereinafter referred to as bus shape or shape of the bus) of the inner surface 2a of the input side disk 2 forming the toroidal-type continuously variable transmission which is structured and operates in the above-mentioned manner is generally formed in a single arc whose radius of curvature does not vary on the way as shown in FIG. 6. Incidentally, the shape of the bus of the inner surface 4a of the output side disk 4 is the same as that of the inner surface 2a of the input side disk 2, and the description thereof is omitted. That is, the bus shape is set as an arc surface having a single curvature which has not only a center of curvature O2a situated at a point nearer to the center axis of the input side disk than the outer peripheral edge of the input side disk 2 having an outside diameter D2 but also a radius of curvature R2a. The diameter (the pitch circle diameter of the inside surface 2a) P.C.D of a circle formed by connecting together the centers of curvature O2a of the buses is smaller than the outside diameter D2 of the input side disk 2 (that is, P.C.D less than D2).
In an operation to grind the inner surface 2a of the input side disk 2 having the above-mentioned shape for finishing the same, as shown in FIG. 7, there is used a grinding wheel 15 in which the shape of the bus of the outer peripheral surface thereof is formed so as to be matched to the shape of the bus of the inner surface 2a, and thus, the inner surface 2a is ground on the grinding wheel 15, while rotating the grinding wheel 15 and input side disk 2. The grinding wheel 15 is rotated about a center axis xcex115 which is inclined by an angle xcex815 toward the input side disk 2 side with respect to a virtual straight line A perpendicular to the center axis xcex12 of the input side disk 2. While the radius of curvature of the bus of the outer peripheral surface of the thus shaped grinding wheel 15 is R2a, the size and installation position of the grinding wheel 15 can be respectively found in the following manner. By the way, the size of the grinding wheel 15 is expressed by its maximum diameter D15 and the diameter d15 of the portion of the outer peripheral surface of the grinding wheel 15 that is used to grind the outer peripheral edge portion of the inner surface 2a. Also, the installation position of the grinding wheel 15 is expressed in the form of a parameter which is associated with an intersection O15-2 between the center axis xcex115 of the grinding wheel 15 and the center axis xcex12 of the input side disk 2. That is, the intersection O15-2 is identical with a point at which a straight line xcex22a connecting the outer peripheral edge portion of the inner surface 2a to the center of curvature O2a of the inner surface 2a intersects with the center axis xcex12 of the input side disk 2. Thus, an intersecting angle xcex82a of straight line xcex22a and the center axis xcex12 is set as the value that represents the installation position of the grinding wheel 15.
xcex82a=sinxe2x88x921{(D2xe2x88x92P.C.D)/2R2a}
d15=(D2/sin xcex82a)xc2x7sin (xcfx80/2xe2x88x92xcex82axe2x88x92xcex815)
D15=d15+2R2a{1xe2x88x92sin (xcfx80/2xe2x88x92xcex82axe2x88x92xcex815)}
In order that the operation to grind the inner surface 2a of the input side disk 2 using the above-mentioned grinding wheel 15 can be executed with high efficiency, it is necessary to increase the peripheral speed of the outer peripheral surface of the grinding wheel 15. To increase such peripheral speed, there are available two methods, that is, in one of them, the outside diameter of the grinding wheel 15 is increased and, in the other, the rotational angular speed of the grinding wheel 15 is increased. Of the two methods, the method of increasing the rotational angular speed of the grinding wheel 15 is limited depending on the capacity of a grinding machine. In this case, it is necessary to employ the method of increasing the outside diameter of the grinding wheel 15. However, in case where the outside diameter of the grinding wheel 15 is increased excessively, there is a possibility that the outer peripheral surface of the grinding wheel 15 can interfere with the near-outer-peripheral portion of the inner surface 2a. That is, since the near-outer-peripheral portion of the inner surface 2a, which is situated to the outside diameter side than the above-mentioned P.C.D., projects on the grinding wheel 15 side. Therefore, in case where the outside diameter of the grinding wheel 15 increases, the outer peripheral surface of the grinding wheel 15 is contacted with the portion of the inner surface 2a that is shifted from the portion of the inner surface 2a to be ground originally. In this state, there is generated excessive resistance with respect to the rotation of the input side disk 2 and grinding wheel 15 and, at the same time, the near-outer-peripheral portion of the inner surface 2a is ground more than necessary, with the result that the shape of the inner surface 2a is shifted from its desired shape.
On the other hand, in JP-A-55-63048, JP-A-7-310796, JP-A-11-82659, and JP-W-5-503342, there is disclosed a technology of forming the bus shape of the inner surface 2a not into a single arc, but into a composite curved surface whose radius of curvature changes on the way with respect to the diameter direction thereof. However, of these publications, the objects of the inventions respectively disclosed in JP-A-55-63048 and JP-A-11-82659 are to reduce loss caused by spin occurring in the contact portion between the inner surface of a disk and the peripheral surface of a power roller. Then, to attain the objects, the radius of curvature of the inner surface is simply made to vary continuously but, in these inventions, there does not exist a technical idea of enhancing the efficiency of an operation to grind the inner surface. Also, the objects of the inventions respectively disclosed in JP-A-7-310796 and JP-W-5-503342 are to reduce the surface pressure of the contact portion between the inner surface of a disk and the peripheral surface of a power roller. Then, to achieve the objects, the radius of curvature of the inner surface is set large on the inside diameter side and small on the outside diameter side. Similarly to the former two inventions, in these inventions, there does not exist a technical idea of enhancing the efficiency of an operation to grind the inner surface.
The present invention aims at eliminating the drawbacks found in the above-mentioned conventional toroidal-type continuously variable transmissions. Accordingly, it is an object of the invention to provide a toroidal-type continuously variable transmission having a structure which can facilitate the grinding of the respective inner surfaces of input side and output side disks to thereby be able to reduce the cost of the toroidal-type continuously variable transmission.
In attaining the above object, according to the invention, there is provided a toroidal-type continuously variable transmission, comprising: two input and output side disks each including an inner surface formed in a concave surface having an arc-shaped cross section, the two disks being concentrically disposed on each other and being rotatably supported independent from each other; an even number of pivot shafts interposed between the two input and output side disks and situated at a torsional relation such that the pivot shafts do not intersect the center axes of the two disks and are perpendicular to the center axes of the two disks; a plurality of trunnions respectively swingable about the pivot shafts; a plurality of displacement shafts respectively projecting from the inner surfaces of the trunnions; and, a plurality of power rollers respectively held between the inner surfaces of the input and output side disks while being rotatably supported respectively on the peripheries of the displacement shafts, each of the power rollers including a peripheral surface formed in a spherical-shaped convex surface, wherein the bus shape of at least one of the inner surfaces of the two input and output side disks is formed in such a manner that the portion of the inner surface situated near to the inside diameter of the disk is formed as a first arc surface with a single arc and the portion of the inner surface situated near to the outside diameter of the disk is formed as a second arc surface with a radius of curvature larger than that of the first arc surface.
In addition, the second arc surfaces of the input and output side disks may be respectively formed as spiral curves.
Further, the above continuously variable transmission may be formed such that the first arc surface has a center of curvature situated nearer to the center axis of the disk than the outer peripheral edge of the disk, the outside diameter of the first arc surface is set substantially identical with P.C.D. which is the diameter of a circle formed by connecting the centers of curvature, and the inside diameter of the second arc surface is set substantially identical with the P.C.D. so as to allow the inner peripheral edge of the second arc surface and the outer peripheral edge of the first arc surface to be smoothly continuous with each other.
In the above-structured toroidal-type continuously variable transmission according to the invention, the operation thereof to transmit power between the input and output side disks as well as the operation thereof to change the transmission ratio between these two disks are similar to those of the previously described conventional toroidal-type continuously variable transmission.
Especially, in the case of the toroidal-type continuously variable transmission according to the invention, even in case where the outside diameter of a grinding wheel for grinding the inner surfaces of the input and output side disks is increased, the outer peripheral surface of the grinding wheel and the near-outside-diameter portions of the inner surfaces of the input and output side disks are hard to interfere with each other. This allows use of a grinding wheel having a larger outside diameter than the conventional one, so that the efficiency of the operation to grind the inner surfaces of the input and output side disks can be enhanced.